The 2009 Family Smoking Prevention and Tobacco Control Act grants the FDA regulatory authority over tobacco-based products. However, the FDA only recently proposed deeming regulations on e-cigarettes. Since their market emergence, e-cigarettes have evolved from first generation products, which look like a conventional cigarette, have few design components and limited nicotine delivery, to second generation products, which do not look like conventional cigarettes, have multiple design components and can achieve cigarette-like levels of nicotine-delivery. Missing from the current literature is a prospective study assessing differences between these two major types of e-cigarettes on smoking behaviors and patterns, nicotine dependence, and on biomarkers of toxicant exposure and effect. Understanding the differential impact of first versus second generation e-cigarettes on smoking as well as measures of proximal health outcomes will help immediately inform the science base needed for the FDA to impose appropriate product specific regulations. The overall aim of the proposed study is to evaluate the effect of switching from conventional cigarettes to either a first or second generation e-cigarette on smoking behavior, product use patterns and continued use, as well as biomarkers of toxicant exposure and effects. To do so, we will randomly assign 453 adult smokers, nave to e-cigarette use, to first generation e-cigarette (G1), second generation e-cigarette (G2), or usual brand cigarette control (UBC). All products will be provided for 12 weeks. Enrollment will be restricted only to smokers not planning to quit in the next 3 months. Prior to randomization, all participants will take part n a lead-in period to assess normal smoking behavior and to allow for stratification on important variables. Follow- ups will occur at 1, 4, 8, 12, 26, and 52 weeks. Our specific aims are to 1) assess the effect of provision of G1 vs. G2 on product switching, abuse liability, number of cigarettes smoked, and perceived nicotine dependence; 2) evaluate changes in biomarkers of harmful tobacco constituent exposure among participants assigned to G1, G2, and UBC; and 3) evaluate changes in biomarkers of toxicant effect among participants assigned to G1, G2, and UBC. This innovative study in its comprehensive examination of all e-cigarette devices as well as their impact on smoking behavior and health outcomes, will provide the scientific foundation the FDA and other agencies need to establish effective regulatory strategies for the manufacture, distribution, and marketing of e- cigarettes.